Jtdy 9, 1874] 



NATURE 



185 



If the tail of the comet consisted of la fine dust not in a state 

 of incindescenoe reflecting or dispsrsiiig the sun's rayj, we 

 should expect its Uglit to ba omplelely polarisad. We seem, 

 tlieiefore, driven to assume, either, i, th\t tiie tail consists of fine 

 incandescent particles ; or, 2, of- particles whose diameter is not 

 small compired with the wavelength ; or, 3, of incandescent 

 gas ; or, 4, possibly of all three of these states combined. 



A. CoWPER RA.NYAUD 



Photographic Irradiation 

 In a letter to Nature, vol. ix. p. 1S3, I gave a short descrip- 

 tion of some experiments on photographic irradiation. The 

 conclusion to which these experiments pointed was that there is 

 a kind of photographic irradiation, caused either by the bright 

 light producing an intense state of chemical activity, which has 

 the power of extending itself in every direction ; or what seems 

 more probable, the parts of the collodion on which the bright 

 light is falling become luminous and reflect light to the surround- 

 ing parts of the sensitive film, and thus extend the chemical 

 change on each side of the true optical boundary line. As the 

 subject is at present under discussion, I send you the results of the 

 following experiments, which seem to support the above con- 

 clusion. In a darkened room a vertical opening iS in. by 

 5 in. was made in the shutter ; over the opening was fixed 

 a piece of paper thick enough to stop most of the light, 

 and only allow as much to pass as would give a decided 

 but not deep photographic impression. Three long, narrow, 

 parallel openings were cut in the paper, one opening was left 

 clear to the sky, the next was covered with one thickness of 

 tissue paper, and the third with two thicknesses of tissue paper. 

 There was thus produced three parallel bars of different bright- 

 ness on a uniform and darker ground. Sensitive wet plates were 

 prepared in the usual way on glass and opaque black plates ; 

 across the front of the plates, anl almost in contact with the col- 

 lodion, was fixed a horizontal bar of thin blackened metal in 

 such a position that it would cross the image of the luminous 

 bars in the camera. The photographs, after exposure, were 

 developed in the usual way, and it was founJ that the shadow 

 cast by the horizontal opaque bar was not bounded by straight 

 lines, but the ends of all the bright bars projected into the 

 shadow, and the brighter the bar the farther it projected. I had 

 no means of measuring accurately the bar and its shadow, but 

 there seems but little doubt that tlie bright bars extended under- 

 neath the opaque bar, whilst the edge of the darker ground at 

 the side of the bright bars gave the correct line of the shadow. 

 Now this extension of the bright bars could not have been caused 

 by tlie reflection from the ba:k of the plate, as this result was 

 always got whetlier glass or opaque black plates were used. Nor 

 could it have been caused by the oblique pencils referred to by 

 Lord Lindsay and Mr. A. C. Ranyard, because, the opaque bar 

 being close to tlie collodion, these pencils could not get under- 

 neath. The natural conclusion seems to be, that this extension 

 of llie bright bars must liave been caused by some molecular 

 reflection taking place in the collodion. This form of irradiation 

 can easily be distinguishe<l from the irradiation produced by 

 reflection from the back of the plate, as the latter is simply a sort 

 of haze surrounding the bright object, extending some distance 

 from it, and gradually fading away, whilst the former extends 

 a very short distance and has a well-marked outline, thougli not so 

 sharp as those parts of the image where there is no irradiation. 

 The irradiation produced by reflection from the back of the 

 plate, and some forms of irradation due to the Imperfections of 

 the lens, though fatal to artist c ]3hotography, yet do not inter- 

 fere much with its scientific valae, as they do not affect the accu- 

 racy of outline, though tliey do affect the clearnesss of the 

 photograph. Molecular irradia'ion, on the other hand, whilst it 

 scarcely affects artistic photograpliy, is fatal to scientific accuracy. 

 Tlie manner of preventing this latter form of irradiation has 

 been already pointed out, namdy, by redu;ing the intensity of 

 the light filling on the sensitive surface to only that necessary to 

 produce a distinct impression. In artistic photography this is 

 almost never possible on account of the different amount of Lglic 

 on the different parts of the subject, while for scientific purposes 

 this may almost always be done. The imperfections of the image 

 due to the lens seem to be as various as the forms of lenses ; oae 

 lens used in the experiments gave a curious double hazy-image of 

 the bright object. When the image is near the centre of the 

 " field " the double image fits over the true image, producing an 

 effect somewhat similar to, and was at first mistaken for the eliect 

 of reflection from the back of the plate. At first this double 

 image was somewhat puzzling, as it always made its appearance 



even when opaque plates were used. The two images were, 

 however, afterwards separated by bringing the true image near 

 the outside of the " field," when the true image and its double 

 were photographed alongside of each other. 



The following simple experiment iUlustrates this molecular 

 form of irradiation, and shows how much tlie definition of the 

 image depends on the nature of the surface which receives it. 

 Take a camera obscura and throw the image on some trans- 

 lucent substance such as opal glass; paint a small part of the 

 glass with some opaque white substance ; bring into the " field" 

 some brilliantly illuminated subject, such as branches of trees 

 against the sky ; examine the iaiige fro.u the lens side of the 

 glass, when it will be found that the image over the opal glass is 

 hazy an i indistinct, whilst the part of the image on the paint 

 shines out brilliant and sharp. John Aitken 



Darroch, Falkirk, N.B.June 16 



Lakes with two Outfalls — A Caution 



Llyn Creigene.v (the larger of the two lakes of that name), 

 situated about five miles .S.W. by VV.of Dolgelly, hs.-.appji-enily 

 two natural outlets— one at the east, the other at the west end of 

 the lake ; both streams ultimately fall into the estuary of the 

 Mawddacli. The two outlets are on nearly the same level, the 

 one at the east end being perhaps a trifle higher than that at the 

 west end. Tlie whole of the waste water at present passes 

 through the western outlet in consequence of an artificial dam of 

 turf having been made across the eastern channel. There are no 

 indications on the ground which would lead anyone to suspect 

 that either of the outlets had been artificially formed ; the gene- 

 ral contour of the surrounding country would rather favour the 

 contrary view. 



I wa-, however, informed last week by a man who had lived 

 eighteen years in the district l/iat he had beat told that originally 

 the only outlet was that at the west end of the Llyn, and that 

 the other outlet had been made many years ago lor the purpose 

 of getting a better supply of water to some mills which then 

 existed, but which do not now exist, on the stream to the east 

 of the lake. If this story prove to be correct it shows ho n im- 

 portant it is to make full inquiries before stating positively that 

 any lake has two natural outfalls. 



From the ordnance map one would imagine that tivo streams 

 issued from Llyn Arenig (five mUes W.N. W. of Bala), but the 

 one shown as starting from the extreme north end of the lake 

 has no existence in fact. George R. Jebb 



Chester, June 3 



A 



FERDINAND STOLICZKA, PH.D. 

 BRIEF telegram from India, which arrived just in 

 time for notice in last week's Nature (vol. x. p. 

 172), announced the death on the 19th ult., at Shayok, 

 between the Karakorurn Pass and Leh in Ladak, of Fer- 

 dinand Stoliczlca, Palaeontologist to the Geological Survey 

 of India, who was returning troni Kashgar and Yarkund 

 with the other members of Mr. Forsyth's mission. 



Thus has passed away, at the early age of thirty-six, a 

 naturalist who, if his lite had been spared, would certainly 

 have_ attained a very high position amongst the leaders 

 of science. Few men have accomplished an equal amount 

 of work in the same brief space ot time. A glance at the 

 Journal and Pio;eedings ot the Bengal Asiatic Society, 

 and the publications ot the Geological Survey of India, 

 especially the " Palaeontologia Indica," will show the won- 

 derful variety of subjects treued by Dr. Stoliczka. In 

 the course of the last ten years, besides geological 

 memoirs on paits of the Western Himalayas and Thibet, 

 he has published numerous papers on Indian mammals, 

 birds, reptiles, amphibia, moUusca, bryozoa, arachnida, 

 coleoptera, and actiiiozoa ; and these papers are no lists 

 of names or mere descaptioas of new species, but they 

 abound with accounts of the life history of the different 

 animals, details of their ana.omy, and remarks on classi- 

 fication, and show that their author was as good an ob- 

 server in the field as he was patient and accurate in the 

 cabinet. His greatest work is undoubtedly his account 

 of the fossil fauna discovered in the Cretaceous rocks of 

 Southern India, in which he proposed the most complete 



